Is there a difference between surfactant-stabilised and Pickering emulsions?

What measurable physical properties allow one to distinguish surfactant-stabilised from Pickering emulsions? Whereas surfactants influence oil/water interfaces by lowering the oil/water interfacial tension, particles are assumed to have little effect on the interfacial tension. Here we perform interfacial tension (IFT) measurements on three different systems: (1) soybean oil and water with ethyl cellulose nanoparticles (ECNPs), (2) silicone oil and water with the globular protein bovine serum albumin (BSA), and (3) sodium dodecyl sulfate (SDS) solutions and air. The first two systems contain particles, while the third system contains surfactant molecules. We observe a significant decrease in interfacial tension with increasing particle/molecule concentration in all three systems. We analyse the surface tension data using the Gibbs adsorption isotherm and the Langmuir equation of state for the surface, resulting in surprisingly high adsorption densities for the particle-based systems. These seem to behave very much like the surfactant system: the decrease in tension is due to the presence of many particles at the interface, each with an adsorption energy of a few kBT. Dynamic interfacial tension measurements show that the systems are in equilibrium, and that the characteristic time scale for adsorption is much longer for particle-based systems than for surfactants, in line with their size difference. In addition, the particle-based emulsion is shown to be less stable against coalescence than the surfactant-stabilised emulsion. This leaves us with the conclusion that we are not able to make a clear distinction between the surfactant-stabilised and Pickering emulsions.

Stringiness of hyaluronic acid emulsions.

OBJECTIVE: Cosmetic emulsions containing hyaluronic acid are ubiquitous in the cosmetic industry. However, the addition of (different molecular weight) hyaluronic acid can affect the filament stretching properties of concentrated emulsions. This property is often related to the "stringiness" of an emulsion, which can affect the consumer's choice for a product. It is thus very important to investigate and predict the effect of hyaluronic acid on the filament stretching properties of cosmetic emulsions. METHODS: Model emulsions and emulsions with low and high molecular weights are prepared and their filament stretching properties are studied by the use of an extensional rheometer. Two different stretching speeds are employed during the stretching of the emulsions, a low speed at 10 µm/s and a high speed at 10 mm/s. The shear rheology of the samples is measured by rotational rheology. RESULTS: We find that filament formation only occurs at high stretching speeds when the emulsion contains high molecular weight hyaluronic acid. The formation of this filament, which happens at intermediate states of the break-up, coincides with an exponential decay in the break-up dynamics. The beginning and end of the break-up of high molecular weight hyaluronic acid emulsions show a power law behaviour, where the exponent depends on the initial stretching rate. At a lower stretching speed, no filament is observed for both high molecular weight and low molecular weight hyaluronic acid emulsions and the model emulsion. The emulsions show a power law behaviour over the whole break-up range, where the exponent also depends on the stretching rate. No significant difference is observed between the shear flow properties of the emulsions containing different molecular weights hyaluronic acid. CONCLUSION: In this work, we underline the importance of the molecular weight of hyaluronic acid on the elongational properties of concentrated emulsions. The filament formation properties, for example the stringiness, of an emulsion is a key determinant of a product liking and repeat purchase. Here, we find that high molecular weight hyaluronic acid and a high stretching speed are the control parameters affecting the filament formation of an emulsion.

OBJECTIF: Les émulsions contenant de l'acide hyaluronique sont omniprésentes dans l'industrie cosmétique. En particulier, l'ajout d'acide hyaluronique (de poids moléculaires différents) peut affecter les propriétés extensionnelles d'un filament d'émulsion concentrée. Cette propriété importante est souvent assimilée à la perception organo-sensorielle "filante/cohésive" d'une émulsion et peut influer sur le choix final du consommateur pour un produit. Il est donc important d'étudier, mais aussi de pouvoir prédire, l'effet de l'acide hyaluronique sur les propriétés d'étirement de filaments d'émulsions cosmétiques. MÉTHODES: Nous avons préparé des émulsions modèles à faible et grands poids moléculaires et étudié leurs propriétés extensionnelles à l'aide d'un rhéomètre extensionnel. Deux vitesses d'étirement différentes sont utilisées, une vitesse faible à 10 µm/s et une vitesse élevée à 10 mm/s. La rhéologie de cisaillement des échantillons est mesurée par rhéologie rotationnelle. RÉSULTATS: Nous constatons que la formation de filaments ne se produit que pour des vitesses d'étirement élevées et lorsque l'émulsion contient de l'acide hyaluronique à grands poids moléculaire. La formation de ce filament, qui se produit à des temps intermédiaires de la rupture, coïncide avec une décroissance exponentielle de la dynamique de rupture. Le début et la fin de la rupture des émulsions d'acide hyaluronique de grands poids moléculaire montrent un comportement en loi de puissance, où l'exposant dépend de la vitesse d'étirement initiale. À une vitesse d'étirement inférieure, aucun filament n'est observé, à la fois pour les émulsions d'acide hyaluronique à grands et faibles poids moléculaires, mais aussi pour l'émulsion modèle ne contenant pas d'acide hyaluronique. Les émulsions présentent un comportement en loi de puissance sur tout le régime de rupture, où l'exposant dépend également de la vitesse d'étirement. Aucune différence significative n'est observée quant aux propriétés d'écoulement de cisaillement des émulsions contenant différents poids moléculaires d'acide hyaluronique. CONCLUSION: Dans cette étude, nous soulignons l'importance du poids moléculaire de l'acide hyaluronique sur les propriétés extensionnelles d'émulsions concentrées. Les propriétés de formation de filaments, ou la perception filante/cohésive d'une émulsion, sont un facteur clé dans l'appréciation d'un produit afin d'assurer un achat répété. Nous démontrons que la présence d'acide hyaluronique à grands poids moléculaires ainsi qu'une vitesse d'étirement élevée, sont les paramètres de contrôle affectant la formation de filaments dans une émulsion.

Fully-biobased UV-absorbing nanoparticles from ethyl cellulose and zein for environmentally friendly photoprotection.

Effective photoprotection is a vital consumer issue. However, there are many concerns regarding the adverse environmental and health impacts associated with current organic and inorganic UV filters. Here, we prepare fully-biobased UV-absorbing nanoparticles from ethyl cellulose (ECNPs) and zein (ZNPs) with encapsulated biobased photoprotectants obtainable from plants and foods (quercetin, retinol, and p-coumaric acid), which have the potential to satisfy both environmental and health issues in photoprotection. We show the ability of ECNPs and ZNPs to be easily tuned compositionally to obtain uniform, broadband UV spectrum absorbance profiles, and prepare transparent UV-absorbing coatings from the ECNPs. We find that the maximum loadings for retinol, quercetin, and p-coumaric acid into the ECNPs are 31 wt%, 14 wt%, and 13 wt% respectively. The ECNP size remains constant (except for the largest loading of retinol, 31 wt%) and the absolute zeta potential increases upon increasing the loading of quercetin and retinol, whereas increasing the loading of p-coumaric acid results in increasing the particle size and a lower absolute zeta potential. We find that quercetin and retinol are effectively retained inside the ECNPs at 64-70% after 72 hours. These results have significant implications for the development of novel photoprotection technologies and functional nanoparticles.